This section provides background information which is not necessarily prior art to the inventive concepts associated with the present disclosure.
Interest in four-wheel drive (4WD) and all-wheel drive (AWD) vehicles has led to development of power transfer systems configured to selectively and/or automatically direct rotary power (i.e. drive torque) from the powertrain to all four wheels of the vehicle. In many 4WD vehicles, the power transfer system includes a transfer case configured to selectively transmit drive torque from the powertrain to the front driveline. In AWD vehicles, the power transfer system includes a power take-off unit (commonly referred to as a “PTU”) configured to selectively transmit drive torque from the powertrain to the rear driveline.
Many current transfer cases are configured to include a rear output shaft interconnecting the powertrain to the rear driveline, a front output shaft interconnected to the front driveline, a transfer assembly drivingly interconnected to the front output shaft, a mode clutch for selectively coupling the transfer assembly to the rear output shaft, and a clutch actuator for controlling actuation of the mode clutch. The mode clutch is operable in a first or “released” state to disconnect the front output shaft from the rear output shaft and establish a two-wheel drive mode (2WD) with all drive torque transmitted from the powertrain to the rear driveline. The mode clutch is also operable in a second or “engaged” state to drivingly connect the front output shaft (via the transfer assembly) to the rear output shaft and establish a four-wheel drive mode (4WD) with drive torque transmitted from the powertrain to both of the front and rear drivelines.
Some “part-time” transfer cases are equipped with a positive-locking type of mode clutch, such as a dog clutch, which can be selectively actuated to shift between the two-wheel drive mode (2WD) and a locked four-wheel drive mode (LOCK-4WD). As an alternative, “active” transfer cases are equipped with an on-demand mode clutch, such as an adaptively-controlled multi-plate friction clutch, configured to automatically control the drive torque distribution between the front and rear drivelines without any input or action on the part of the vehicle operator, so as to provide an on-demand four-wheel drive mode (AUTO-4WD) in addition to the two-wheel drive mode (2WD). Typically, active transfer cases include a power-operated clutch actuator that is interactively associated with an electronic traction control system having a plurality of vehicle sensors. The power-operated clutch actuator regulates the magnitude of a clutch engagement force applied to the multi-plate friction clutch based on vehicular and/or road conditions detected by the sensors, thereby adaptively regulating the drive torque distribution ratio between the front and rear drivelines.
Vehicle scenarios exist in which the transfer case clutch can become damaged. One known scenario occurs when a sudden and relatively large differential speed results between the front output and rear output shafts of the actively-controlled multi-plate transfer case. Such a difference in speed between the front and rear output shafts can occur during simultaneous depression of a throttle pedal and brake pedal, wherein the front tires are stopped from rotation via sufficiently sized front brakes, while the rear tires, particularly on a low coefficient of friction road surface, can be permitted to rotate. In such scenario, the transfer case software may identify the rear wheel slip and attempt to equalize the front and rear wheel speeds by increasing the transfer case clutch torque capacity, up to the maximum clutch torque capacity. As such, the combination of high torque capacity and high level of slip can quickly increase the operating temperature of the transfer case clutch, resulting in damage thereof.